Clutch size and breeding success in relation to nest-box size in Tengmalm's owl Aegolius funereus

Clutch size and breeding success was related to nest-box size in 215 nests of Tengmalm's owl in the Kauhava region, western Finland in 1966–82. In vole peak years, but not in other years, mean clutch size was positively correlated with the bottom area of the nest-box, increasing on average by 0.005 eggs cm⁻². In successful nests there were fewer nestlings in small and medium-sized than in large boxes. Breeding was most successful in thick-walled boxes with a small entrance diameter. The optimal box for Tengmalm's owl has the following dimensions: bottom area 20 × 20 cm, entrance diameter 8 cm, and wall thickness at least 3 cm.
Hypotheses suggested to explain the "area effect" based on breeding data of hole-nesting passerines do not hold good for Tengmalm's owl. Since there were more stored prey animals in medium-sized and large boxes than in small boxes, I suggest that the "area effect" is related to food availability, which may determine how sensitive the species is to different bottom areas of the boxes, at least in areas with cyclic food (e.g. northern Europe).     

Fasting endurance and cold resistance without hypothermia in a small predatory bird: the metabolic strategy of Tengmalm's owl,Aegolius funereus

The energetic adaptations of non-breeding Tengmalm's owls (Aegolius funereus) to temperature and fasting were studied during the birds' autumnal irruptions in western Finland. Allometric analysis (including literature data and two larger owl species measured in this study) indicates that the basal metabolic rate of owls is below the mean level of non-passerine birds. However, the basal metabolic rate of the 130-g Tengmalm's owl (1.13 W) is higher than in other owls of similar size. This is probably related to its northern distribution and nomadic life history. Relative to its size, Tengmalm's owl has excellent cold resistance due to effective insulation (lower critical temperature +10°C, minimum conductance 0.19 mW·cm^-2·°C^-1). Radiotelemetric measurements of body temperature showed that the level of body temperature is lower than for birds in general (39.4°C at zero activity) and that the amplitude of the diurnal cycle is also low (0.2–0.6°C). In contrast to many other small birds, Tengmalm's owls do not enter hypothermia during a 5-day fast at thermoneutrality or in cold. Moreover, while the metabolic rate per bird shows the expected mass-dependent decrease, the mass-specific rate decreases only slightly during the fast. In line with this, there was no decrease in the plasma triiodothyronine concentration during the fast in the owl, whereas a dramtic drop was observed in the pigeon and Japanese quail that were used as a reference. Despite this, the owl has an excellent capacity for fasting because of its ability to accumulate extensive fat depots and its low overall metabolic rate. Fasting reduced evaporative water loss to 50% of that in the fed state. Calculations show that the oxygen consumption observed in fasting birds would involve a production of metabolic water barely sufficient to compensate for evaporative water loss. The threat of dehydration may thus set a limit to the decrease in metabolic rate in fasting owls (owls rely totally on water either ingested with food or produced metabolically). We conclude that the metabolic strategy in Tengmalm's owl is largely dictated by an evolutionary pressure for fasting endurance. With the restrictions set by small body size and water economy, this bird has apparently taken these adaptations to an extreme. The constraints that preclude hypothermia, which could increase the capacity for fasting even more, remain unknown.Abbreviations BM body mass - BMR basal metabolic rate - EWL vaporative water loss - MR metabolic rate - T₃ triiodothyronine - T _a ambient temperature - T _b body temperature - VO₂ oxygen consumption     

Pygmy owl Glaucidium passerinum and the usage of ultraviolet cues of prey

Birds rely mainly on their vision when foraging. Many diurnal raptors use ultraviolet (UV) vision and ultraviolet‐reflecting vole scent marks to find suitable hunting areas, whereas nocturnal owls seem to lack this ability. We studied if the diurnal pygmy owl Glaucidium passerinum that uses voles and birds as its food can detect vole scent marks using UV‐vision. We conducted a laboratory experiment with eleven owls. Each individual owl had four options to choose from: (1) scent marks with UV light, (2) scent marks without UV light, (3) clean arena with UV light and (4) clean arena without UV light. The owls scanned the scent mark arena more often in the presence of UV light than other arenas. However, owls did not spend more time above the UV arena. We suggest that pygmy owls can detect near UV and use UV to gain information about prey like other diurnal raptors.     

Does predation maintain tit community diversity?

European tits of the genus Parus constitute a complex group of coexisting boreal birds. Here we present a survey of the distribution of three coniferous-living Parus species and one of their main predators, the pygmy owl ( Glaucidium passerinum ), on nine isolated islands in Scandinavia. On all islands the coal tit ( Parus ater ) is the sole tit species when the pygmy owl is absent. The two larger species, the willow tit ( P. montanus ) and the crested tit ( P. cristatus ), only coexist with the coal tit when pygmy owls are present. We suggest that the coexistence of willow tits, crested tits and coal tits is the result of a combination of competition for food and predator-safe foraging sites. The smaller coal tit is superior in exploitation competition for food, while the two larger species have an advantage in interference competition for predator-safe foraging sites. The association between the distribution of the pygmy owl and the two larger tit species on isolated islands in Scandinavia is consistent with the idea that the pygmy owl is a keystone predator.     

Resource partitioning among forest owls in the River of No Return Wilderness,Idaho

Summary We studied resource partitioning among the forest owls in the River of No Return Wilderness, Idaho, during the winter and spring of 1980 and 1981. The owl assemblage consisted of five abundant species: pygmy (Glaucidium gnoma), saw-whet (Aegolius acadicus), boreal (A. funereus), western screech (Otus kennicottii), and great-horned (Bubo virginianus). Long-eared (Asio otus) and flammulated (O. flammeolus) owls were rarely observed. Information from the literature supplemented our data to describe the pattern of resource partitioning. Stepwise discriminant function analysis and multivariate analysis of variance revealed differences in macrohabitat and microhabitat. The saw-whet, boreal, western screech, and great-horned owls all preferred mammalian prey but exhibited habitat differences. They also differed in activity periods and food habits. The pygmy owl, a food and habitat generalist, foraged diurnally more than the other species and took a higher proportion of brids. The flammulated owl used areas within the territories of other owl species but specialized on forest insects. The observed pattern of resource use was interpreted to result from environmental factors, morphological limitations and interspecific competition. Differences in food and activity time, we suggest, result from environmental factors and differences in owl morphology, while present-day interspecific competition may be important in shaping habitat use. Experiments will be necessary to determine the causal factors responsible for segregation among the forest owls.     

Refuge sites of voles under owl predation risk: priority of dominant individuals?

In an aviary experiment, we studied whether body size or habitatfamiliarity of field voles (Microtus agrestis) affected predationrisk by Tengmalm's owls (Aegolius funereus). In the field, wecompared the body size of field voles snap-trapped in good (covered)and poor (open) habitats in 1992 and 1994 to determine whetherthere were habitat-related differences in the body size of voles.In the aviary, large individuals occupied the good habitat significantlymore than small individuals both in the control (owl not present)and experimental treatments (owl present). Furthermore, habitat-familiarvoles inhabited the good habitat more than habitat-unfamiliarvoles did when an owl was present Our field data were consistentwith our aviary data: larger field voles were more frequentlyfound in good habitats than in poor habitats. In the aviary,Tengmalm's owl predation risk was higher for small and habitat-unfamiliarvoles. This suggests that large field voles may have priorityto sheltered habitats. Furthermore, habitat familiarity mayplay a central role in avoiding risky habitats.     

Why do territorial male Tengmalm's owls fail to obtain a mate?

Non-breeding may occur because non-breeders are immature or somehow physiologically incapable of breeding, or because of a lack of resources (e.g. food resources, mating partners) needed to breed. There is, however, a lack of experimental evidence on whether bachelor males possessing territories and nest-sites are able to breed when supplemented with extra food or provided with mating partners. In vole-eating Tengmalm's owl, Aegolius funereus, we provided supplementary food and transferred females in nest-boxes of non-breeding males. Bachelor males that we supplemented with food did not attract mates at a higher frequency than unfed control males, which suggests that a lack of food did not influence the ability to attract a mating partner. In contrast, bachelor males presented with a female seemed to breed more frequently than bachelor males in the control group without mate addition. This suggests that scarcity of females may be an important reason for the high proportion of non-breeding males in the population (c. 25%) and excludes the possibility that non-breeding males are physiologically unable to breed. The operational sex ratio of the owl population at the time of mating may be male-biased, and some males may thus remain unpaired. Habitat and nest-box quality also seemed to be lower among bachelors than among breeding males. Received: 22 September 1997 / Accepted: 29 December 1997     

Habitat composition as a determinant of reproductive success of Tengmalm's owls under fluctuating food conditions

The effect of landscape composition on the breeding success of vole-eating Tengmalm's owl ( Aegolius funereus ) was studied in western Finland at five different spatial scales (250–4000 m) around the nests during two consecutive three-year population cycles of voles. Landscape composition had strongest effects on owl breeding in the decrease phase of vole cycles. Significant variation in owl breeding occurred along the productivity gradient from farmland predominated areas to barren hinterland. Owls tended to produce earlier clutches on territories predominated by agricultural areas in increasing vole years. A similar trend was observed in the decreasing phase of the vole cycle; owls breeding on barren hinterland seemed to delay breeding compared to owls breeding near agricultural areas. Surprisingly, nestling survival and fledgling production in the decreasing phase declined steeply with increasing proportion of farmland. Clutch size was not significantly related to landscape composition. The number of fledglings decreased with increases in clear-cut and sapling areas in the decrease phase. During the declining years of vole abundance nestling survival increased from western farmland areas towards the eastern outlying district. These results indicate sudden summer decline of vole populations on farmland predominated habitats. This is probably due to that the number of vole-eating predators, and hence their impact on vole populations is apparently higher in farmland areas than on forested hinterland. This finding gives support for the 'spill-over' hypothesis, which states that predators and their exploitation tends to 'spill over' from luxuriant habitats to the barren habitats.     

Gradients in population fluctuations of Tengmalm's owl Aegolius funereus in Europe

Summary I studied the importance of geographical location, snow cover and food to the fluctuations in 30 breeding populations of Tengmalm's owl (Aegolius funereu) in Europe. Cyclicity indices were positively related both to latitude and longitude, but within Fennoscandia they were better correlated with snow cover. Population fluctuations increased northward, while food niche breadth and degree of site tenacity decreased northwards.Microtine fluctuations become more pronounced northwards and are more synchronized, while number of alternative prey increases southwards. These factors promote instability in North European and stability in central European owl populations. Furthermore, snow conditions were more important within Fennoscandia, since this small owl cannot hunt voles protected by deep snow. Environmental predictability and diversity of available food for Tengmalm's owl increase southwards in Europe. Thus, the owl is a resident generalist predator of small mammals and birds in central Europe and adopts a partial migration strategy (males being resident and females nomadic) in South and West Finland, changing its habits to nomadic microtine specialist in areas with pronounced vole cycles (in northern Fennoscandia). These changes fit well with the recent suggestion that gradients in density variations of small rodents are related to sustainable numbers of generalist predators.     

Hatching asynchrony and brood reduction in Tengmalm's owl <Emphasis Type="Italic">Aegolius funereus</Emphasis>: the role of temporal and spatial variation in food abundance

Hatching asynchrony is the consequence of birds initiating incubation before clutch completion. It has been suggested that variation in hatching asynchrony in owls is extensive, and therefore they should be excellent objects to study the effects of spatio-temporal variation in food abundance on this phenomenon. We examined how abundance and predictability of food affected hatching asynchrony in Tengmalm's owl Aegolius funereus (Linnaeus), which mainly feeds on voles which fluctuate in 3- to 4-year cycles in northern Europe. Hatching span averaged 6-7 days (range 0-13 days) and increased with clutch size. Food supply did not directly influence levels of hatching asynchrony but it influenced indirectly via marked among-year changes in clutch size. During the decrease phase of the vole cycle the proportion of hatchlings producing fledglings decreased with asynchrony, suggesting that chick mortality was most common among asynchronous broods when food became scarce. This finding is consistent with Lack's brood reduction hypothesis, i.e. that if food becomes scarce during the nestling period the youngest nestlings would die first without endangering the survival of the whole brood.     

Climate change and perishable food hoards of an avian predator: Is the freezer still working?

Changing climate can modify predator–prey interactions and induce declines or local extinctions of species due to reductions in food availability. Species hoarding perishable food for overwinter survival, like predators, are predicted to be particularly susceptible to increasing temperatures. We analysed the influence of autumn and winter weather, and abundance of main prey (voles), on the food‐hoarding behaviour of a generalist predator, the Eurasian pygmy owl (Glaucidium passerinum), across 16 years in Finland. Fewer freeze–thaw events in early autumn delayed the initiation of food hoarding. Pygmy owls consumed more hoarded food with more frequent freeze–thaw events and deeper snow cover in autumn and in winter, and lower precipitation in winter. In autumn, the rotting of food hoards increased with precipitation. Hoards already present in early autumn were much more likely to rot than the ones initiated in late autumn. Rotten food hoards were used more in years of low food abundance than in years of high food abundance. Having rotten food hoards in autumn resulted in a lower future recapture probability of female owls. These results indicate that pygmy owls might be partly able to adapt to climate change by delaying food hoarding, but changes in the snow cover, precipitation and frequency of freeze–thaw events might impair their foraging and ultimately decrease local overwinter survival. Long‐term trends and future predictions, therefore, suggest that impacts of climate change on wintering food‐hoarding species could be substantial, because their ‘freezers’ may no longer work properly. Altered usability and poorer quality of hoarded food may further modify the foraging needs of food‐hoarding predators and thus their overall predation pressure on prey species. This raises concerns about the impacts of climate change on boreal food webs, in which ecological interactions have evolved under cold winter conditions.     

Barred Owl Space Use and Habitat Selection in the Eastern Cascades,Washington

ABSTRACT Competition with barred owls (Strix varia varia) is an important factor contributing to the continued decline of threatened northern spotted owl (Strix occidentalis caurina) populations in the Pacific Northwest, USA, but basic information on habitat selection and space use patterns of barred owls is lacking for much of the region. We investigated space use and habitat selection by tracking radiotagged barred owls in the Eastern Cascade Range of Washington, USA, from 2004 to 2006. We surveyed for barred owls across the 309-km² study area and confirmed presence of barred owl pairs at 21 sites. We collected movement data on 14 barred owls from 12 sites. Mean annual 95% fixed-kernel home-range size was 194 ha for females (n = 4, SD = 70) and 288 ha for males (n = 5, SD = 114). Home ranges were located more frequently than expected in areas with low topographic position, gentle slopes, large overstory tree-crown diameter, high normalized difference vegetation index (NDVI), overstory tree canopy closure >72%, and a moderate amount of solar insolation. Within home ranges, areas that had large tree-crown diameters, low topographic positions, and gentle slopes were used more frequently than expected. The resource selection function we developed for barred owls in our study area indicated that barred owls used areas with the combination of low values for topographic position and slope and higher values for NDVI, solar insolation, and an interaction term for canopy closure and tree-crown diameter. In comparison to published information on northern spotted owls, barred owls used areas with similar canopy closure and tree size classes, but barred owl home ranges were much smaller and more concentrated on gentler slopes in valley bottoms. This information may contribute to the development of management practices that maintain forest characteristics appropriate for spotted owl habitat and prey in areas where spotted owls are least likely to be excluded by territorial barred owls in the Eastern Cascades of Washington.     

Maternal transfer of androgens in eggs is affected by food supplementation but not by predation risk

Mothers may affect the future success of their offspring by varying allocation to eggs and embryos. Allocation may be adaptive based on the environmental conditions perceived during early breeding. We investigated the effects of food supplementation and predation risk on yolk hormone transfer in the pied flycatcher Ficedula hypoleuca. In a food supplementation experiment, females were food‐supplemented prior to and during egg‐laying and androgen concentrations were measured throughout the laying order. Predation risk was investigated in three different studies combining both correlative data, where flycatchers bred in close proximity to two different predator species that prey upon adult flycatchers (either Tengmalm's owl Aegolius funereus or pygmy owl Glaucidium passerinum), and an experimental manipulation, where flycatchers were exposed to cues of a nest predator (least weasel Mustela nivalis). Females receiving food supplementation laid eggs with lower concentrations of androstenedione (A4) than females not receiving food supplements. Yolk testosterone (T) concentration showed the same pattern but the difference was not statically significant. Testosterone (but not A4) concentration increased within clutches, from the first to the last egg, independently of the food supplementation. Females breeding under high predation risk did not differ from control females in their yolk androgen levels (A4, T or progesterone). However, concentrations of A4 tended to be lower in the proximity of pygmy owls, which could indirectly increase offspring survival after fledging. Food supplementation during egg‐laying seems to have a stronger impact on maternal transfer of androgens than predation risk. Food availability and predation risk could differentially affect the trade‐offs of androgen allocation for the offspring when raised in good vs. dangerous environments.     

Food supplementation and predation risk in harsh climate: interactive effects on abundance and body condition of tit species

Food availability and predation risk can have drastic impacts on animal behaviour and populations. The tradeoff between foraging and predator avoidance is crucial for animal survival and will strongly affect individual body mass, since large fat reserves are beneficial to reduce starvation but may increase predation risk. However, two‐factor experiments simultaneously investigating the interactive effects of food and predation risk, are still rare. We studied the effects of food supplementation and natural predation risk imposed by pygmy owls Glaucidium passerinum on the abundance and fat reserves of tit species in boreal forests of north Europe, from January to March in 2012 and in 2013. Food supplementation increased the number of individuals present in a given forest patch, whereas the level of predation risk had no clear impact on the abundance of tit species. The stronger impact of food supply respect to predation risk could be the consequence of the harsh winter conditions in north Europe, with constant below‐zero temperatures and only few (5–7 h) daylight hours available for foraging. Predation risk did not have obvious effects on tit abundance but influenced food consumption and, together with food supplementation, affected the deposition of subcutaneous fat in great tits Parus major. High owl predation risk had detrimental effects on body fat reserves, which may reduce over‐winter survival, but the costs imposed by pygmy owl risk were compensated when food was supplemented. The starvation–predation tradeoff faced by great tits in winter may thus be mediated through variation in body fat reserves. In small species living in harsh environment, this tradeoff appeared thus to be biased towards avoidance of starvation, at the cost of increasing predation risk.     

Inclusion body disease in a great horned owl.

The carcass of a great horned owl (Bubo virginianus), which had been found moribund in southern Ontario, was presented for necropsy. Throughout the liver and spleen were numerous white foci 1-2 mm in diameter; also noted were white plaques in the mucosae of the pharyngeal papillae and intestine. Results of light and electron microscopic studies and experimental transmission to two captive great horned owls suggested that this was a herpvirus disease similar and possibly indentical to the owl disease reported by other workers in Wiconsin and Australia.     

The Invasion of Barred Owls and its Potential Effect on the Spotted Owl: a Conservation Conundrum

The spotted owl (Strix occidentalis) is a threatened species in many areas of its western North American range. Concomitant with its decline has been a rapid invasion of its range and habitat by barred owls (Strix varia), a native species that was restricted, until relatively recently, to eastern North America. We assess the theoretical potential for negative interactions between these two owls by examining size dimorphism and ecological relationships within various owl assemblages throughout the world. We then review the anecdotal, natural history, modeling, and experimental evidence that suggest barred owls may negatively affect spotted owls with at least a potential for the competitive exclusion of spotted owls by barred owls throughout all or part of the former’2019;s range. While it is widely accepted that barred owls are either causing or exacerbating declines of spotted owl populations, there are confounding factors, such as habitat loss and bad weather that also may contribute to declines of spotted owls. Both theory and empirical information suggest that barred owls are likely to have negative effects on spotted owl range and density, but the degree of the impact is not predictable. There is a conservation conundrum here, in that the barred owl is a native species that has expanded its range westwards, either naturally or with a degree of human facilitation, and now constitutes a major threat to the viability of another native species, the threatened spotted owl. We propose that only through carefully designed experiments involving removal of barred owls will we be able to determine if recent declines in spotted owl populations are caused by barred owls or by other factors. It is rare in conservation science that replicate study areas exist for which we also have long-standing demographic information, as is the case with the spotted owl. Removal experiments would take advantage of the wealth of data on spotted owls, and allow ecologists to assess formally the impacts of an invasive species on a threatened species, as well as to suggest mitigation measures.     

The relationship between intra–guild diet overlap and breeding in owls in Israel

Even though intra-guild predators frequently prey on the same species, it is unclear whether diet overlap between two predators is a source of interspecific competition or whether predators simply use the same abundant prey resource. We measured the extent to which the diets of barn owls (Tyto alba) and long-eared owls (Asio otus) in Israel overlap and examined whether yearly differences in diet overlap correlate with barn owl breeding success. Pianka’s index of niche overlap was positively related to barn owl population size but not to its breeding success. The number of breeding barn owls was higher when long-eared owls consumed more rodents, suggesting that diet overlap most likely increased when rodents became more abundant. Therefore, in Israel, when these two owl species prey more often on rodents, their diets are more similar and interspecific competition is reduced. Unlike sympatric populations in Europe, in years when rodents are less abundant in Israel long-eared owls switch to hunting alternative prey (e.g., birds), perhaps to avoid competition with barn owls.     

Increased autumn rainfall disrupts predator–prey interactions in fragmented boreal forests

There is a pressing need to understand how changing climate interacts with land‐use change to affect predator–prey interactions in fragmented landscapes. This is particularly true in boreal ecosystems facing fast climate change and intensification in forestry practices. Here, we investigated the relative influence of autumn climate and habitat quality on the food‐storing behaviour of a generalist predator, the pygmy owl, using a unique data set of 15 850 prey items recorded in western Finland over 12 years. Our results highlighted strong effects of autumn climate (number of days with rainfall and with temperature <0 °C) on food‐store composition. Increasing frequency of days with precipitation in autumn triggered a decrease in (i) total prey biomass stored, (ii) the number of bank voles (main prey) stored, and (iii) the scaled mass index of pygmy owls. Increasing proportions of old spruce forests strengthened the functional response of owls to variations in vole abundance and were more prone to switch from main prey to alternative prey (passerine birds) depending on local climate conditions. High‐quality habitat may allow pygmy owls to buffer negative effects of inclement weather and cyclic variation in vole abundance. Additionally, our results evidenced sex‐specific trends in body condition, as the scaled mass index of smaller males increased while the scaled mass index of larger females decreased over the study period, probably due to sex‐specific foraging strategies and energy requirements. Long‐term temporal stability in local vole abundance refutes the hypothesis of climate‐driven change in vole abundance and suggests that rainier autumns could reduce the vulnerability of small mammals to predation by pygmy owls. As small rodents are key prey species for many predators in northern ecosystems, our findings raise concern about the impact of global change on boreal food webs through changes in main prey vulnerability.     

Review of the effects of barred owls on spotted owls

Barred owls (Strix varia) are forest-dwelling owls, native to eastern North America, with populations that expanded westward into the range of the spotted owl (Strix occidentalis). Barred owls exert an overwhelmingly negative influence on spotted owls, thereby threatening spotted owl population viability where the species co-occur. In this review, we provide an overview of the barred owl's range expansion and detail and synthesize previously published literature on spotted and barred owls within the range of the spotted owl as related to potential future outcomes for the northern spotted owl (S. o. caurina). We include research on diet, habitat use and selection, effects of barred owls on spotted owl demography and behavior, hybridization with spotted owls, parasites, contemporary management, and future research needs for spotted owl populations given continued barred owl expansion throughout western North America. Our literature review and synthesis should provide managers with the information necessary to develop strategies that mitigate deleterious effects of barred owls at local and landscape scales. © 2019 The Wildlife Society.